The invention relates to a method for producing a container from a thermoplastic plastic film, the container having a hollow floor that forms a double-walled standing base, with the use of a molding tool, and to the molding tool for executing the method.
German Patent No. DE-PS 1 291 469 discloses a generic apparatus, in which containers having a hollow floor or bottom and a double-walled standing base are deep-drawn from a thermoplastic plastic film, and then punched out of the film. The gap between the two walls of the standing base is sealed, because the container floor or bottom is pressed against a shoulder provided around the entire perimeter of the mold when the hollow floor is formed. In this production method, after the hollow floor and the standing base are formed, the ejector floor is retracted, so that, as described in column 14, lines 46–60, of the reference the standing base can shrink inward until the ejector floor impacts the standing base, which now has a smaller diameter, as the ejector floor executes its second upward movement to eject the container. The reduction in the diameter is necessary because of the undercut, which is formed by the shoulder Pos 170, and over which the standing base must be guided as it is ejected. FIGS. 11 and 12 of this document illustrate this desirable, but unrealistic, method particularly well.
The above-described method has various drawbacks. The desired, necessary cooling of the standing base is interrupted and greatly delayed when the ejector floor is retracted, because the floor is usually cooled, which effects the necessary, but no longer guaranteed, cooling. The standing base is cooled further through contact with the cooled container mold. If the standing base shrinks inward, however, it releases from the container mold, immediately halting the cooling process. For the above-cited reasons, the cooling process is very slow, so only a small cycle number is possible with the thermoforming machine due to the poor cooling, on the one hand, and the double lifting movement of the ejector floor, on the other hand, which additionally presupposes a corresponding control. The shrinkage of the standing base is strongly dependent on the plastic material, and the described method cannot be implemented with plastics having a low shrinkage.
FIG. 12 of the reference illustrates particularly clearly that the ejected containers are positioned in an unstable manner on the ejector floor. Ejecting and stacking containers that have already been punched out in the molding tool is therefore highly susceptible to faults, and not applicable in certain stacking methods, e.g., when the molding table is pivoted with the one half of the molding tool for stacking the containers. The containers are then ejected at an incline or horizontally, and would fall from the ejector floor in this method. It is not possible to additionally apply suction to the containers.
European Published Patent Application No. EP 0 259 214 A1 discloses a method for producing containers having a hollow floor or bottom and a standing base, in which the hollow floor and thus the standing base are formed by a two-part mold floor comprising the ejector floor and spreading pliers. The spreading pliers are spring-mounted, so that during the production of the hollow floor, the pliers and the ejector floor execute a lifting movement that forms the hollow floor and the standing base. The spreading pliers press the inside wall of the standing base against the conical container wall, but do not effect a pinched seal comparable to that of the method according to DE-PS 1 291 469. Presently, however, a seal between the two walls of the standing base is required for several reason, for example, to render the container reusable, or extend the shelf life of highly-perishable contents by preventing a product from entering this gap.
A further disadvantage of this known method is that the container is unmolded by means of a further upward movement of the ejector floor without a prior change in the position of the spreading pliers. Consequently, the ejector floor must withdraw the double-walled standing base from the gap between the spreading pliers and the container mold, which requires a certain force that may lead to a permanent deformation of the container. It is practically impossible to cool the inside surface of the standing base, because the spreading pliers Pos 16 would have to be cooled, which would be very complicated. No indirect cooling of the spreading pliers by a cooled ejector floor is provided, and would not be very effective in view of the small contact surfaces between the two parts.
Moreover, the spreading pliers are costly to produce, and the molding tool is susceptible to heavy wear, depending on the per-cycle friction between the spreading pliers and bordering parts in the region of the surfaces Pos 18, 20 and 21, 22, respectively. The pliers have slotted elements for executing their function. Dirt or plastic filings can enter the slots, thus debilitating the spreading pliers and necessitating an involved cleaning of the molding tool.
It is known from German Patent reference No. DE 30 24 683 to produce containers having a conical, welded standing base. For unmolding the containers, which have an undercut, parts of the molding tool are embodied to be displaced perpendicular to the center axis. This type of molding tool cannot be arranged in multiple rows.
Moreover, in a known method of the Assignee, the hollow floor and the standing base are formed by a displaceable ejector floor that is surrounded by an annular sealing bell over the width of the two walls of the standing base. The two components together execute the ejection stroke. This method does not permit a seal between the two walls of the standing base, because it is not possible to unmold an undercut here.